High Side Pressure Regulation For Transcritical Vapor Compression System

ABSTRACT

An expensive expansion device may be eliminated in favor of a less expensive pressure regulator in a CO 2  vapor compression system such as is used in a bottle cooler or small-capacity air conditioner, refrigerator, or other system.

CROSS-REFERENCE TO RELATED APPLICATION

Benefit is claimed of U.S. patent application Ser. No. 60/663,960, filedMar. 18, 2005, and entitled “High Side Pressure Regulation forTranscritical Vapor Compression System”, the disclosure of which isincorporated by reference herein as if set forth at length.

BACKGROUND OF THE INVENTION

The invention relates to refrigeration. More particularly, the inventionrelates to beverage coolers.

As a natural and environmentally benign refrigerant, CO₂ (R-744) isattracting significant attention. In most air-conditioning operatingranges, CO₂ systems operate in transcritical mode. FIG. 1 schematicallyshows transcritical vapor compression system 20 utilizing CO₂ as workingfluid. The system comprises a compressor 22, a gas cooler 24, anexpansion device 26, and an evaporator 28. The exemplary gas cooler andevaporator may each take the form of a refrigerant-to-air heatexchanger. Airflows across one or both of these heat exchangers may beforced. For example, one or more fans 30 and 32 may drive respectiveairflows 34 and 36 across the two heat exchangers. A refrigerant flowpath 40 includes a suction line extending from an outlet of theevaporator 28 to an inlet 42 of the compressor 22. A discharge lineextends from an outlet 44 of the compressor to an inlet of the gascooler. Additional lines connect the gas cooler outlet to expansiondevice inlet and expansion device outlet to evaporator inlet.

The major difference between transcritical and conventional operation isthat heat rejection in the gas cooler is in the supercritical regionbecause the critical temperature for CO₂ is 87.8° F. Consequently,pressure is not solely dependent on temperature and this opensadditional control and optimization issues for system operation.

For a fixed gas cooler discharge temperature, as the high side pressureis increased, the exit enthalpy of the refrigerant decreases, yielding ahigher differential enthalpy through the gas cooler. The capacity of thegas cooler is a function of the mass flowrate of refrigerant and theenthalpy difference across the gas cooler. For a beverage cooler, theevaporator may be essentially at the cooler interior temperature. It istypically desired to maintain this temperature in a very narrow rangeregardless of external condition. For example, it may be desired tomaintain the interior very close to 37° F. This temperature essentiallyfixes the steady state compressor suction pressure.

For a fixed compressor suction pressure, as the high side pressureincreases, the amount of energy used by the compressor increases, andthe volumetric efficiency of the compressor decreases. When thevolumetric efficiency of the compressor decreases, the flowrate throughthe system decreases. The balance of these two counteracting effects istypically an increase in gas cooler capacity as the high side pressureis increased. However, above a certain pressure the amount of capacityincrease becomes very small. Because the expansion device is usuallyisenthalpic, the evaporator capacity will also typically increase as thehigh side pressure increases.

The energy efficiency of a vapor compression system, the Coefficient ofPerformance (COP), is usually expressed as a ratio of the systemcapacity to the energy consumed. Because an increase in pressuretypically produces both a higher capacity and a higher energyconsumption, the balance between the two will dictate the overall COP.Therefore, there is typically an optimal pressure which yields thehighest possible performance.

An electronic expansion valve is usually used as the device 26 tocontrol the high side pressure to optimize the COP of the CO₂ vaporcompression system. An electronic expansion valve typically comprises astepper motor attached to a needle valve to vary the effective valveopening or flow capacity to a large number of possible positions(typically over one hundred). This provides good control of the highside pressure over a large range of operating conditions. The opening ofthe valve is electronically controlled by a controller 50 to match theactual high side pressure to the desired set point. This pressurecontrol strategy involves a fairly high cost valve, a sophisticatedcontroller 50, and a sensor 52 for measuring the high side pressure.This equipment adds a significant amount of cost to the CO₂ vaporcompression system, causing the CO₂ vapor compression system to be lessattractive compared to an HFC system.

It is possible to use a fixed expansion device in a transcritical vaporcompression system, but this approach has limitations which may cause aloss of performance or functionality. During steady state operation, afixed expansion device (e.g., a fixed orifice or capillary tube) canwork well to regulate the system high side pressure to a near optimumpressure. During pulldown, when the system is started and theevaporation temperature and pressure can be very high, the flowratethrough a fixed speed and displacement compressor can become relativelyhigh. This high flowrate can cause the high side pressure to exceed asafe limit.

SUMMARY OF THE INVENTION

An expensive expansion device may be eliminated in favor of a lessexpensive pressure regulator in a CO₂ vapor compression system such asis used in a bottle cooler or small-capacity air conditioner,refrigerator, or other system. The potential for overpressurization maybe reduced by using an inexpensive, multi-step fixed expansion devicebased on one or more solenoid valves.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a prior art vapor compression system.

FIG. 2 is a schematic of a first inventive CO₂ vapor compression system.

FIG. 3 is a schematic of a second inventive CO₂ vapor compressionsystem.

FIG. 4 is a schematic of a third inventive CO₂ vapor compression system.

FIG. 5 is a schematic of a fourth inventive CO₂ vapor compressionsystem.

FIG. 6 is a schematic of a fifth inventive CO₂ vapor compression system.

FIG. 7 is a schematic of a sixth inventive CO₂ vapor compression system.

FIG. 8 is a schematic of a seventh inventive CO₂ vapor compressionsystem.

FIG. 9 is a side schematic view of a display case including arefrigeration and air management cassette.

FIG. 10 is a view of a refrigeration and air management cassette.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The current invention relates to high-side pressure optimization for aCO₂ vapor compression system. For HVAC & R products which do not havebroad operating envelopes, the optimal high side pressures for alloperating conditions do not vary much. Therefore, a fixed expansiondevice (e.g., an orifice or capillary tube) can be used to regulate thehigh side pressure to a preset constant value for all steady stateoperating conditions of the CO₂ vapor compression system. The presetvalue should be determined such that the CO₂ vapor compression systemcan achieve the best overall Coefficient of Performance (COP) for theentire operating envelope. Using a fixed expansion device cansignificantly reduce the cost of the pressure control components in aCO₂ vapor compression system.

For pulldown conditions, the compressor flowrate will be significantlyhigher than during steady state conditions. The high-side pressureshould be optimized such that the pulldown cooling capacity of the CO₂vapor compression system can be maximized, but the flow through thepressure regulator does not exceed the flow through the compressor (sothat the system pressure becomes too great). This optimal high-sidepressure for maximizing capacity is usually higher than the optimalhigh-side pressure for maximizing the overall COP. However, because thecompressor flowrate is much higher during pulldown conditions thanduring steady state conditions, the expansion device may be configuredto have a larger flow capacity during pulldown conditions. A simplemulti-position expansion device may provide this. There are a number ofways through which this can be achieved through the use of solenoidvalves to enable a two or more position pressure control system.

The following examples reflect modifications of the basic system ofFIG. 1. Accordingly, the same reference numerals are used to identifythe compressor 22, gas cooler 24, and evaporator 28. In anyreengineering or remanufacturing situation, these components may beidentical to those of the baseline system or may be further modified.FIG. 2 shows a system 60 in which the refrigerant flow path 62 is splitinto two parallel branches/segments 64 and 66 between the gas cooler 24outlet and evaporator 28 inlet. The first branch 64 has a first fixedexpansion device 68. The second branch 66 includes, in series, asolenoid valve 70 and a second fixed expansion device 72. Although thesolenoid valve 70 is shown upstream of the second fixed expansion device72, this order may be reversed. The exemplary solenoid valve 70 has twosettings/conditions. One setting/condition is a fully closed conditionin which no flow may pass along the second branch 66. The secondsetting/condition is a fully open condition allowing flow to passthrough the second branch 66 with a minimal pressure loss across thesolenoid valve 70.

During steady state operating conditions, when the compressor flowrateis relatively low, the solenoid valve 70 is kept fully closed. Duringpulldown conditions, the compressor flowrate is relatively high. Inorder to avoid overpressurization during pulldown, the solenoid valve 70is opened, allowing flow through the second fixed expansion device 72.The combination of both expansion devices 68 and 72 regulates thehigh-side pressure to avoid overpressurization while still deliveringgood system performance.

In operation, a pulldown condition may be detected by means of one ormore temperature sensors 75 and pressure sensor 74 coupled to acontroller 76 coupled to control the solenoid valve 70. The controller76 may also be coupled to the compressor and/or fan(s) to control theirrespective operation. For ease of illustration, the sensor andcontroller are not illustrated in the following examples although theymay be present.

FIG. 3 shows a system 80 wherein the refrigerant flow path 82 has twosegments/branches 84 and 86 in parallel upstream of a first fixedexpansion device 88. The first branch 84 includes a solenoid valve 90.The second branch 86 includes a second fixed expansion device 92. Duringsteady state operating conditions, the solenoid valve 90 is closed toprevent flow along the first branch 84. The second branch 86 acts as abypass with restricted flow passing through the second fixed expansiondevice 92 before then passing through the first fixed expansion device88. During pulldown conditions, the solenoid valve 90 is open, allowingan essentially unrestricted flow along the first branch 84. A smalladditional flow may flow along the second branch 86, with the combinedflow then passing through the first expansion device 88. In alternativeembodiments, the first expansion device 88 may be upstream of thebranching rather than downstream. Control methods and components (notshown) of this system and those discussed below may be similar to thoseof the system 60.

FIG. 4 shows another system 100 wherein the flow path 102 has first andsecond segments/branches 104 and 106 between the gas cooler andevaporator. A fixed expansion device 108 is located in the first branch104. A solenoid valve 110 is located in the second branch 106. Thesolenoid valve 110 combines aspects of a solenoid valve and a fixedexpansion device. Specifically, the open condition may still berelatively restricted compared with the open condition of the solenoidvalve 90. Therefore, the pulldown pressure drop through the solenoidvalve 110 is significant and the high-side pressure of the system iscontrolled to the preset constant optimal value by the combination ofthe solenoid valve 110 and the fixed expansion device. For steady stateoperation, the solenoid valve 110 is fully closed and all flow passesthrough the expansion device 108.

FIG. 5 shows a branch-less system 120 in which, along the flow path 122,a solenoid valve 124 and fixed expansion device 126 are located inseries. The solenoid valve 124 combines aspects of the solenoid valveand a fixed expansion device differently from the valve 1 10 of FIG. 4.Specifically, the valve element (e.g., the solenoid plunger) of thesolenoid valve 124 may have a small orifice so that its closed conditionis only a partially closed condition. The open condition, however, is anessentially fully open condition with low pressure drop. Accordingly,during steady state operating conditions, the solenoid valve 124 is inits closed condition passing a relatively low flow and creating asubstantial pressure drop (individually and combined with the expansiondevice 126). In the steady state condition, the solenoid valve is open,permitting the flow rate to be dictated essentially solely by theexpansion device 126. As with the other systems, the series order may bereversed.

FIG. 6 shows a system 140 combining aspects of the systems 80 and 120.Specifically, the flow path 142 has two segments/branches 144 and 146 inparallel upstream of a first fixed expansion device 148. The firstbranch 144 includes a solenoid valve 150. The second branch 146 includesa fixed expansion device 152. The exemplary solenoid valve 150 may,similar to the solenoid valve 124, have a closed condition that is onlypartially closed. During pulldown conditions, the solenoid valve 150 isopen. During steady state conditions, the valve 150 is closed. In thesteady state condition, there is a relatively small flow along each ofthe branches. During pulldown conditions, a larger flow may pass alongthe first branch 144, with a residual flow along the second branch 146.

FIG. 7 shows another system 160 wherein the flow path 162 includes asolenoid valve 164 that combines solenoid valve and orifice functions.Specifically, the element of the solenoid valve 144 includes an orificeso that the closed condition is only partially closed. During steadystate conditions, the valve 144 is in its closed condition with theorifice passing the relatively small flow. During pulldown conditions,the valve is open so that a larger flow is passed.

FIG. 8 shows a system 180 wherein the flow path 182 includessegments/branches 184 and 186 between the gas cooler and the evaporator.A solenoid valve 188 and 190 is located in each of the branches. Theelements of these solenoid valves may include orifices. Independentcontrol over the valves may provide more than two alternative effectiveflow restrictions. For example, with different size orifices, the twovalves provide up to four different effective restrictions. A minimalrestriction may be present with both valves open. A maximal restrictionmay be present with both valves closed. A pair of intermediaterestrictions may be achieved with one of the valves closed and the otheropen. To provide a more than trivial difference amongst the three leastrestrictive conditions, the conduit of the branches may be sized or thevalve sized or additional restriction may be present so that with onlyone valve open there is not essentially free flow. An alternativeembodiment could feature such valves in series rather than parallel.

A variety of sensor and/or user inputs may be used to control thesolenoid valve(s). Direct measurement of the high-side pressure may bemade by the sensor 74. When this pressure exceeds one or more associatedthresholds, the controller 76 may cause the valve(s) to assume anassociated relatively free-flow condition. Alternatively or in additionto high-side pressure measurement would be sensor 74, input may bereceived from an air temperature sensor. The exemplary sensor 75 may bepositioned to be exposed to air in or from the cooler interior (e.g., tothe flow 36 upstream of the evaporator 28). The sensor 75 may form partof a control thermostat. Accordingly, use of such a sensor alone maypermit cost savings through the elimination of the pressure sensor 52 or74.

For fixed speed and displacement compressor, the flow through the systemis a direct function of the density of the refrigerant entering thecompressor and, to a lesser extent, the pressure ratio of thecompressor. The inlet density is a direct function of the saturationtemperature and superheat of the refrigerant. These, in turn, are directfunctions of the air temperature, system size, and charge. For a simplesystem, these parameters may be determined in the design stage as afunction of air temperature flowing through the evaporator. Acorrelation can be produced which matches the evaporator air temperatureto the refrigerant inlet density. In operation, the solenoid valve(s)would remain in the open position until the output of the evaporatortemperature sensor 75 drops below a predetermined value. When thishappens, the solenoid valve or one of the solenoid valves is closed.This can be repeated for systems having multiple solenoid valves furtherreducing the effective expansion orifice area as the temperature dropsso as to maintain a mere optimal pressure in the high pressure portionof the system.

If a high-side pressure is directly measured (e.g., by the sensor 74) adifferent correlation may be used. The optimal high-side pressure may beknown as a function of evaporator temperature and, optionally, theambient temperature. The solenoid valve or valves may be actuated tomaintain the pressure within certain limits.

FIG. 9 shows an exemplary cooler 200 having a removable cassette 202containing the refrigerant and air handling systems. The exemplarycassette 202 is mounted in a compartment of a base 204 of a housing. Thehousing has an interior volume 206 between left and right side walls, arear wall/duct 216, a top wall/duct 218, a front door 220, and the basecompartment. The interior contains a vertical array of shelves 222holding beverage containers 224.

The exemplary cassette 202 draws the air flow 34 through a front grillein the base 224 and discharges the air flow 34 from a rear of the base.The cassette may be extractable through the base front by removing oropening the grille. The exemplary cassette drives the air flow 36 on arecirculating flow path through the interior 206 via the rear duct 210and top duct 218.

FIG. 10 shows further details of an exemplary cassette 202. The heatexchanger 28 is positioned in a well 240 defined by an insulated wall242. The heat exchanger i28 is shown positioned mostly in an upper rearquadrant of the cassette and oriented to pass the air flow 36 generallyrearwardly, with an upturn after exiting the heat exchanger so as todischarge from a rear portion o the cassette upper end, a drain 250 mayextend through a bottom of the wall 242 to pass water condensed from theflow 36 to a drain pan 252. A water accumulation 254 is shown in the pan252. The pan 252 is along an air duct 256 passing the flow 34 downstreamof the heat exchanger 24. Exposure of the accumulation 254 to the heatedair in the flow 34 may encourage evaporation.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, when implemented as a remanufacturing of an existing system orreengineering of an existing system configuration, details of theexisting configuration may influence details of the implementation.Accordingly, other embodiments are within the scope of the followingclaims.

1. A refrigeration system (60; 80; 100; 120; 140; 160; 180) comprising:a compressor (22) for driving a refrigerant along a flow path (62; 82;102; 122; 142; 162; 182) in at least a first mode of system operation; afirst heat exchanger (24) along the flow path downstream of thecompressor in the first mode; a second heat exchanger (28) along theflow path upstream of the compressor in the first mode; and a pressureregulator (68; 72; 88; 92; 108, 110; 124, 126; 148, 152; 164; 188; 190)in the flow path downstream of the first heat exchanger (24) andupstream of the second heat exchanger (28) in the first mode.
 2. Thesystem of claim 1 wherein the pressure regulator comprises a non-valvefixed orifice expansion device (68; 72; 88; 92; 108; 126; 148; 152). 3.The system of claim 1 wherein the pressure regulator comprises anon-valve fixed orifice expansion device (126; 148) in series with asolenoid valve (124; 150) with a valve element having an orifice.
 4. Thesystem of claim 1 wherein the pressure regulator comprises a non-valvefixed orifice expansion device (88; 148) in series with a parallelcombination of a solenoid valve (90; 150) and bypass conduit (86; 146).5. The system of claim 1 wherein there are first and second suchpressure regulators in parallel.
 6. The system of claim 1 wherein: therefrigerant comprises, in major mass part, CO₂; and the first and secondheat exchangers are refrigerant-air heat exchangers.
 7. The system ofclaim 1 wherein: the first and second heat exchangers and compressor areremovable from a housing of the system as a unit without need topreviously empty contents of the system.
 8. The system of claim 1wherein: the refrigerant consists essentially of CO₂; and the first andsecond heat exchangers are refrigerant-air heat exchangers each havingan associated fan, a first mode air flow across the first heat exchangerbeing an external to external flow and a first mode airflow across thesecond heat exchanger being a recirculating internal airflow.
 9. Thesystem of claim 1 being a cooler containing: a plurality of beveragecontainers in a 0.3-4.0 liter size range.
 10. The system of claim 9wherein the cooler is selected from the group consisting of: acash-operated vending machine; a transparent door front, closed back,display case; and a top access cooler chest.
 11. A refrigeration systemcomprising: a compressor driving a CO₂-based refrigerant along a flowpath in at least a first mode of system operation; a first heatexchanger along the flow path downstream of the compressor a second heatexchanger along the flow path upstream of the compressor; and means inthe flow path downstream of the first heat exchanger and upstream of thesecond heat exchanger for expanding the refrigerant in the absence of anelectronic expansion device.